Sintered molded parts of iron materials are usually fabricated by pressing powder in axial presses to form green compacts or compressed powder charges, and these are subsequently sintered by largely standardized processes. In these cases, sinter densities of about 90% of the theoretical density are achieved. The sinter density can only be conditionally improved by means of known additional processes, unless other major disadvantages are accepted. Correspondingly, the mechanical strength properties of such sintered molded parts remain inferior to those of molded parts of smelted, 100% dense materials.
The cost advantages of fabrication without generating any chips, that is, loose particles removed from the body, make the sintering technique favorable for use in the production of molded parts. With respect to the dimensions achieved in powder pressing, the finished parts have good dimensional stability and narrow, reproducible dimensional tolerances. Furthermore, on account of the residual porosity existing after sintering, sintered molded parts can be brought, very precisely, to a predetermined reference dimension by pressing.
There are, then, many processes which have become known for bringing conventionally sintered molded parts to at least a uniform, approximately theoretical, i.e., 100%, material density, those conventionally sintered parts being typically of uniform material and, as usual, affected by residual porosity. Powder forging is one of the proposed processes, which does not quite achieve full density. Hot isostatic pressing is a further suitable process which is, however, very elaborate due to the necessary enveloping of the powder and sintered body and is, therefore, unsuitable for mass-produced parts.
The sintering HIP process is a modification of the HIP process, by means of which residual porosities in a sintered part can likewise be eliminated. However, this process also displays many of the restrictions as noted above.
All these processes are used with the aim of improving not only the mechanical properties of sintered molded parts, but also, for example, their corrosive properties. A disadvantage of all these processes is that such a refined sintered molded part becomes a "blank" which has to be further mechanically finished and which to this extent differs significantly from conventionally fabricated sintered parts. Conventionally fabricated sintered molded parts, which are optionally calibrated subsequent to sintering in pressing operations, are generally components which are ready to install.
Furthermore, processes are known for making moldings composed of different materials in different regions, that are as dense as possible and consequently mechanically strong in all regions, of which at least one of these regions is a sintered body.
For instance, DE-A1 22 58 310, entitled "Sinter-eisen-Formteil sowie Verfahren und Sinterkachel zu seiner Herstellung" ("Sintered-iron molded part as well as process and sintering tile for its production"), describes a way by which, during the sintering process, a molded part pressed from iron material "is brought into connection with an agent from which austenite-forming elements diffuse into the surface of the molded part, at least at the sintering temperatures". This causes a material refinement in the surface region, with the aim of improving the surface wear resistance. The finished sintered-iron molded part has porosity in all regions; even in the diffusion region, the molded part has at least "closed porosity" with altogether a maximum of about 95% material density.
According to the teaching of DE-A1 23 10 536, "Verfahren zur Herstillung von Gegenstaanden aus Verbundmetall" ("Process for producing articles from composite metal"), a core part, produced by melt-metallurgy that is thereafter completely dense molded, is placed in the center of a container, with the intermediate space between core and container wall filled with a metal powder The "known" i e container-enclosed, composite is exposed in an autoclave to such high compaction pressures and temperatures on all sides that its density on all sides comes "into the range of 100% of the theoretical density". The composite thus obtained is subsequently forged or rolled out, for example. According to what is claimed, powder densities of more than 95% of the theoretical density are achieved by this process. The composite body is dense in its entirety. This allows composites for use, for example, for the cited application of a milling tool, teeth or other irregular cutting surfaces, wherein the core consists of relatively tough and easily machined metals, while the boundary zones consist of extremely hard material.
DE 30 07 008 describes a wear-resistant part for internal-combustion engines which comprises a basic body of a smelted iron or steel material and an iron-containing sintered body intimately bonded with the basic body by sintering. What is essential for the invention of DE 30 07 008 is the iron alloy proposed for the sintered body. This process too serves the purpose of producing parts "which are distinguished by high toughness in the interior of the body and a particularly high abrasion resistance, at least in a section of their surface."
According to DE-A2 20 50 276, for producing a workpiece with a wear-resistant surface, a wear-resistant hard metal powder is pressed onto and sintered onto a steel basic body. Unlike the sintering of iron materials, hard metal can be produced approximately 100% dense on account of the molten binder phase during sintering. The finished composite body is uniformly dense. A disadvantage in this case is the strong sintering shrinkage, which rules out the production of molded parts in narrowly toleranced reference dimensions without chip-forming, requiring additional finishing. Other disadvantages include material brittleness and material costs.
All the known prior publications have in common the fact that material composites are created by joining together individual material regions by using the sintering technique. The finished material composites have, as far as possible, a high density throughout, in the best case approaching theoretical 100% density. The finished material composites also display individual molded part regions having different mechanical properties, but always with high wear and strength values in the region of surface zones.
Accordingly, in further development of the said prior art, the object of the present invention is to attain, in the case of molded parts of iron materials produced by means of the sintering technique, a high mechanical strength which can be achieved for 100% dense materials in the zones of the molded part which are correspondingly loaded, but which nevertheless allows subsequent calibrating of the sintered molded part.